Ectoparasites and particularly salmon lice infest salmonids such as Atlantic salmon (Salmo salar L) and trout (Salmo trutta) in seawater, and have caused substantial economic losses in the fish farming industry in Norway, Scotland, Canada and Chile. The estimated annual losses in recent years are in the order of one billion Norwegian krone (NOK) for the salmon farming industry in Norway alone. In addition, with the expansion of the fish farming industry and the inevitable increased abundance of salmon lice in the marine environment, sea-lice infestations are being regarded as an escalating threat also to wild fish stocks. There are also reasons for being concerned about the environmental impacts of the pesticides currently used against salmon-lice, a concern creating negative attitudes in the society to the salmon farming industry. Accordingly, sea-lice infestations is not merely a severe economic problem to the fish farming industry itself, it has created reasons for grave concern about the implications it may have for coastal ecosystems and the communities where fish farming is being conducted.
Anti-parasitic drugs such as organophosphates (trichlorvos, dichlorvos and azamethiphos) and permethrins (cypermethrin and deltamethrin) have been used to bath-treat sea-lice infested salmon. Other treatments include use of chitin synthesis inhibitors such a diflubenzuron and teflubenzuron and more recently the anti-parasitic drug emamectin benzoate, administered in the feed. Due to the inevitable process of resistance development against chemically synthesized pesticides, these compounds will most probably gradually lose their antiparasitic efficacy. It is generally accepted that such chemotherapeutic treatments are not satisfactory, both with regard to environmental acceptability and from an efficacy point of view. Hence, alternative treatments and prevention strategies are urgently needed. (M. Costello 2006. Trends in Parasitology, Vol. 22 No. 10, 475-483).
The use of hydrogen-peroxide against sea-lice is a notable idea tried out in practice for some years. Hydrogen peroxide will be split by catalase and cause formation of oxygen bubbles inside the sea-lice. Due to the increased buoyancy caused by these bubbles, the sea-lice will be forced to detach from the skin and float to the water surface. The method is very stressful for the fish, however, and hydrogen peroxide does not kill the lice—they may settle again. Due to practical difficulties and variable efficacy results, the method is not in widespread use.
Wrasse is a predator fish biologically specialized to feed on salmon-lice they pick off salmon skin. The use in practice of this biological principle is an attractive and ecologically sound method of sea-lice control in salmon farms, tried out in Norway during the last 10-15 years. However, there are notable limitations to large-scale implementation of the method. The wrasse fish used so far do not survive winter conditions in salmon farms, the size of the predator fish population must be adjusted according to the size of the growing salmon, and the biggest salmon swim too fast for the smaller size wrasse fishes to succeed in picking lice.
U.S. Pat. No. 5,401,727 disclose a process for stimulating the immune system of aquatic animals of the class Osteichthyes and subphylum Crustacea comprising administering an effective amount of a yeast cell wall glucan composed of glucopyranose units linked by predominantly beta-1,3 glucosidic bonds, having at least one branch therefrom of glucopyranose units linked by beta-1,6 glycosidic bonds. Additionally the invention provides a process for enhancing the effect of vaccines by administering an effective amount of the described yeast cell wall glucan along with vaccine antigens. Further this invention also provides a process of obtaining a glucan particularly effective for stimulating the immune system of aquatic animals of the class Osteichthyes and subphylum Crustacea.
Most attention in recent years has been paid to mobilizing immune mechanisms to render the fish more resistant to lice infestation. Two principles have been in focus, the first is to enhance innate immune mechanisms by known immune stimulants such as beta-1,3/1,6-glucan, the second is to mobilize adaptive immunity in the fish by injecting vaccines comprising unique antigens present in the particular sea-lice strains infesting farmed salmon. Both strategies are promising, but the use of beta-1,3/1,6-glucan is apparently closer to being implemented in commercial scale than the vaccination strategy, as commented on below.
When added to salmon feed, MACROGUARD® (an immune modulating beta-1,3/1,6-glucan) induces notable protection against salmon-lice infestation (Fish Farming International, July 2004, Vol. 34, No. 7, page 3), most likely by enhancing innate immune mechanisms in the skin and mucosa of the fish. Such mechanisms may include mucous immunoglobulins, lysozyme, complement factors and immune cells that guard tissue surfaces and attack intruding parasites. It is well known among those skilled in the art that beta-1,3/1,6-glucan triggers defence reactions by interacting with Toll-like receptors (TLRs) and with the highly specific beta-1,3/1,6-glucan receptor designated dectin. Beta-1,3/1,6-glucan is one out of many microbial structures recognized by so-called Pattern Recognition Receptors (PRRs) present in immune cells of fish and higher animals. The PRRs that recognize the beta-1,3/1,6-glucan structure have during evolution been involved in dealing with fungal intruders and in eliciting an adequate response to such infections. But activation of the PRRs specific for beta-1,3/1,6-glucan, although specifically “designed” by Nature to cope with fungal infections, also give rise to enhanced protection against virus and bacteria—and against parasites. There are at least 9 different TLRs on immune cells, designed to recognize and respond to 9 different unique bacterial and viral structures (Aderem A and RJ Ulevitch. Toll-like receptors in the induction of the innate immune response. Nature 2000; 406:782-787). However, no specific TLR for parasite structures has hitherto been found.
The fact that only one commercial vaccine exists against an ectoparasite (the cattle tick vaccine) is a reflection of the underlying difficulties associated with successful parasite protection by the adaptive immune system. The commercial tick vaccine comprises a concealed antigen that induces the production of antibodies interfering with the ticks' ability to digest blood. A corresponding strategy has been followed in Norway in research-based attempts to develop a salmon-lice vaccine. It has been demonstrated that vaccination with a concealed antigen from salmon lice induces production of antibodies that end up in the digestive organ of blood sucking salmon-lice, Challenge experiments with salmon-lice of the same type as that used in the vaccine formulation (antigens present in female salmon lice), have demonstrated that these antibodies indeed have a protective effect. The results are therefore promising, although the vaccination projects using purified salmon-lice specific antigens are still at a research stage. When new vaccine candidates targeting vital functions in sea-lice have been found, it remains to produce such vaccines commercially. If such vaccines are to be based on concealed antigens present in salmon-lice, it will take time to develop them because molecular biological screening of genes coding for the most promising targets needs to be carried out first. While waiting for the eventual great break-through in parasite vaccine development, every attempt should be made to combine principles that contribute to reducing the salmon louse problem in the fish farming industry.